This invention relates generally to image processing and image reconstruction based on acquired raw image data. In particular, improving the emission image quality in combined imaging systems such as CT-PET and CT-SPECT which use a CT image for multiple purposes including diagnostic review, fusion review with the PET, and calibration and correction of the PET data for effects including attenuation and scatter.
Raw image data from various diagnostic medical systems, such as Computed Tomography (CT), Positron Emission Tomography (PET) systems, and Single Photon Computed Tomography (SPECT) systems are acquired for diagnostic purposes. The CT PET and SPECT systems are configured to support numerous scanning and reconstruction modes. The associated reconstruction algorithms are complex and computationally intensive. Users of the diagnostic medical systems desire an improvement in image quality, to minimize the time required to generate images based on raw image data, and to improve the reliability of the reconstruction process. By decreasing the amount of time needed to generate the desired images from raw image data, images can be evaluated sooner and patient through-put may be improved.
Accordingly, at least one known PET-CT system and one known SPECT-CT system, utilizes data that are generated by the CT apparatus to generate an attenuation correction of the PET or SPECT scan data. Specifically, a plurality of emission attenuation correction factors are derived from CT data that is generated during a CT scan, wherein the CT system is specifically configured to generate data to be utilized for the CT attenuation correction factors. A method of converting the CT Hounsfield units into emission attenuation factors is described in U.S. Pat. No. 6,856,666. In this document, the term CTAC is used to denote the map of emission Attenuation Coefficients which are derived from the CT images.
For example, PET scan data is acquired and processed as a sequence of axial frames along the length of the patient. PET images are typically reconstructed at axial intervals corresponding to sub-multiples of the average axial detector spacing, for instance, half of the detector spacing. CT axial images from multislice detectors are typically reconstructed with axial intervals corresponding to multiples of the nominal detector interval. For example the PET slices are reconstructed at intervals of approximately 3.3 mm, corresponding to half the detector spacing, with a nominal slice thickness of approximately four millimeters. The CT axials images are reconstructed at intervals corresponding to multiples of the nominal detector interval of 0.625 mm. CT helical scans are normally reconstructed at intervals equal to the slice thickness, but on the GEHC PET-CT the interval for helical scans can be set to any value. For the purposes of PET attenuation correction, the CT interval is set equal to the PET interval. Accordingly, known CT/PET systems generate CT attenuation correction factors, by first generating a set of CT slices that approximately match the thickness of the PET slice, at axial locations matching the PET slices.
While appropriate for generating the CT attenuation correction factors utilized to generate a PET image, this relatively thick CT slice is generally not useful for medical imaging purposes. Specifically, CT images useful for medical diagnosis, are typically less than four millimeters thick to facilitate enhancing image resolution and thus medical diagnosis of the patient. Therefore, while generating a relatively thick CT slice is beneficial to generate the attenuation correction factors used to generate the PET image, the relatively thick CT slice may not have the image quality or resolution to allow an operator to perform a medical diagnosis of the patient using the generated CT image.